1. Field of the Invention
The present invention relates to an imaging apparatus that can communicate with a lens unit to transmit and receive information and a lens unit that can be detachably mounted on the imaging apparatus.
2. Description of the Related Art
A clock synchronous type serial communication method (i.e., a synchronous serial communication method) has been conventionally employed for communications to be performed between a camera and a lens unit in an interchangeable lens system. A conventional technique discussed in Japanese Patent Application Laid-Open No. 9-304804 includes performing synchronous serial communications at a first baud rate so that a camera can communicate with a mounted lens unit even when the lens unit is an old type. If communication contents reveal that a mounted lens unit is a new type, then the method includes starting synchronous serial communications at a second baud rate that is higher than the first baud rate.
Further, the above-described conventional method includes switching a communication terminal circuit from an open drain type to a Complementary Metal Oxide Semiconductor (CMOS) type that is suitable for high-speed communications, in the above-described baud rate switching operation. When the above-described conventional technique is employed for a camera communication system, the camera can operate at a lower speed when it is combined with an old type lens unit and can operate at a higher speed when it is combined with a new type lens unit.
According to the conventional synchronous serial communication method, a lens microcomputer analyzes data received during an interval between a preceding communication and a succeeding communication and sets data to be transmitted to the camera in the next communication based on the received data. To this end, a waiting time (Busy) is provided to enable the lens microcomputer to execute required processing.
During the waiting time (Busy), the lens microcomputer does not accept any communication data transmitted from the camera. In other words, a camera microcomputer is kept in a standby state until the lens microcomputer is released from the “Busy state” even when the clock frequency is raised. Therefore, communication performances of the camera microcomputer cannot be improved sufficiently.
On the other hand, the lens microcomputer frequently receives a communication interrupt from the camera. When an interrupt signal is received from the camera, the lens microcomputer prioritizes communication interrupt processing over other processing to output a “Busy signal” and subsequently cancel the “Busy signal.” Therefore, communication performances of the lens microcomputer cannot be improved sufficiently.